1. Field of the Invention
The present invention relates to a thermal dye-transfer type recording sheet, and more particularly to a recording sheet for use in the thermal dye-transfer type recording method, in which a substrate coated with a coloring material is used, the coloring material layer is brought into contact with a recording sheet and the coloring material is transferred to the recording sheet by heating by a thermal head or the like.
2. Prior Art
In the thermal recording method there is widely adopted a method in which a heat-sensitive recording paper having formed thereon a recording layer to be colored under heating by a physical or chemical change is brought into contact with a thermal head, whereby a record of a desirable color is obtained on the heat-sensitive recording paper. The heat-sensitive recording method, however, is defective in that coloration contamination is readily caused in a heat-sensitive recording paper because of pressure or heat unavoidably applied to the heat-sensitive recording paper during storage or at the time of handling and that the manufacturing cost of such heat-sensitive recording paper is high.
As means for overcoming the above defects of the conventional heat-sensitive recording method, there has been proposed a method as disclosed in the Japanese Patent Application Laid-Open Specification No. 15446/76. It discloses that a substrate, such as paper or resin film, is coated with a coloring material which is solid or semi-solid at room temperatures, the coloring material coated on the substrate is brought into contact with a recording sheet and the coloring material on the substrate is selectively transferred to the recording sheet by heating by a thermal head to perform recording. The reason why transfer of the coloring material coated on the substrate to the recording sheet is effected is that the coloring material or a binder containing the coloring material is molten, evaporated or sublimated by heating and adhesion or absorption of the coloring material to the recording sheet is thus caused. The coloring material customarily used in this recording method is a dispersion of a dye or pigment in a binder such as a wax. Even if this dispersion type coloring material is brought into contact with a recording sheet in normal conditions, transfer of the coloring material does not occur. Transfer of the coloring material takes place first when assembly of the coloring material and the recording sheet is heated at, for example, 60 to 300.degree. C. or about 500.degree. C. in some cases. Sublimable dyes having a sublimation temperature of 60 to 300.degree. C., for example, disperse dyes of the nitro, azo, quinoline and anthraquinone types are preferably used as the coloring material. Accordingly, this recording method is characterized in that plain paper can be used as the recording sheet.
However, when plain paper is used as the recording shet in the above recording method, the color density of the obtained record is low and fading of the color with the lapse of time is conspicuous. In other words, a recording sheet suitable for use in this recording method has not been developed.